1. Field of the Invention
The present invention relates to an improvement of a bearing unit for wheel for supporting a wheel of an automobile as well as a rotary body for braking such as a rotor or a drum, and to an improvement of a method of manufacturing such a bearing unit for wheel.
2. Description of the Related Art
A wheel 1 constituting a traveling wheel of an automobile and a rotor 2 constituting a disc brake as a braking apparatus are rotatably supported by a knuckle 3 which constitutes a suspending apparatus with a structure, for example, as shown in FIG. 8. To be specific, a circular supporting hole 4 is formed in the knuckle 3. An outer ring 6 being a stationary ring constituting a bearing unit 5 for the wheel, to which the present invention is applied, is fixed to within the supporting hole 4 by use of a plurality of bolts 7. On the other hand, the wheel 1 and the rotor 2 are fixedly connected to a hub 8 as a component of the wheel bearing unit 5 through a plurality of studs 9 and a plurality of nuts 10.
Outer ring double raceways 11a, 11b each serving as a stationary-side raceway surface are formed in an inner circumference of the outer ring 6, and a joint flange 12 is provided on an outer circumference thereof. The thus configured outer ring 6 is fixed to the knuckle 3 by connecting the joint flange 12 to the knuckle 3 by the bolts 7.
On the other hand, a mounting flange 13 is formed at a portion projecting from an outer end opening of the outer ring 6, and the flange 13 is formed at a part of the outer circumference of the hub 8. (Herein, the term “outer” implies an outside portion in the width direction in a state of being assembled to the automobile, that is, the left side in FIGS. 1 through 4, 6 and 8. In contrast, the term “inner” implies a central portion in the width direction in the state of being assembled to the automobile, that is, the right side in FIGS. 1 through 4, 6 and 8). The wheel 1 and the rotor 2 are fixedly connected in combination to one side surface of the flange 13 (in general, the outer side as illustrated) through the studs 9 and nuts 10. Further, an inner ring raceway 14a is formed in a portion, facing to the more external raceway 11a of the outer ring double raceways 11a, 11b, of the outer peripheral face of an intermediate portion of the hub 8. Moreover, an inner ring is fixedly connected onto a small-diameter stepped portion 15 formed at the inner end of the hub 8. An inner ring raceway 14b formed at the outer peripheral of the inner ring 16 is disposed opposite to the more internal raceway 11b of the outer ring double raceways 11a, 11b. 
Balls 17, 17 each defined as a rolling member are provided in a rollable manner by pluralities between the outer ring raceways 11a and 11b and the inner ring raceways 14a and 14b, in a state of being held by retainers 18, 18. With this structure being provided, a double-row angular ball bearing with a back face combination is structured, the hub 8 is rotatably supported in the inner side of the outer ring 6 so as to support radial and thrust loads. Between the inner peripheries of both ends of the outer ring 6, the outer periphery of the intermediate portion of the hub 8, and the outer periphery of the inner end of the inner ring 16, seal rings 19a, 19b are provided respectively to intercept a space holding the balls 17, 17 from an outside space. Further, since the illustrated embodiment is the wheel bearing unit 5 for a drive wheel (a rear wheel of FR car and RR car, a front wheel of FF car and all wheels of 4WD car), the hub 8 has a spline hole 20 formed at the center thereof. Then, a spline shaft 22 of a constant speed joint 21 is inserted into the spline hole 20.
When the rolling bearing unit 5 for the wheel is used, the outer ring 6 is fixedly connected to the knuckle 3 and at the same time the wheel 1 combined with a tire (not shown) and the rotor 2 are fixedly connected to the fitting flange 13 of the hub 8, as shown in FIG. 8. The rotor 2 of these components is combined with a support and a caliper (both not shown) which are fixed to the knuckle 3, thus constituting a disc brake for braking. When braking operation, a pair of pads sandwiching the rotor 2 are pressed against both side surfaces of the rotor 2.
It is known that vibrations called as a judder followed by unpleasant noises often occur when braking the automobile. As one of various causes of these vibrations, known is an ununiform state of frictions between the side surface of the rotor 2 and the lining of the pad, however, deflections of the rotor 2 are also known as other causes. Deflection of the rotor 2 is also a big cause. That is, the side surface of the rotor 2 must be substantially normal to the rotation center of the rotor 2. It is, however, difficult to obtain a perfect perpendicularity due to an unavoidable manufacturing error. As a result, while the car is running, the side surface of the rotor 2 will inevitably deflect in the direction of the rotation axis (left and right directions in FIG. 8) though being more or less. When the deflection is increased (displacing amount in the left and right directions in the same), and the linings of the pair of pads are pressed against both sides of the rotor 2 for braking, the judder is produced.
For restraining the judder generated by such causes, it is important to suppress (improve) the deflection (axial deflection) in the axial directions of the side surfaces of the rotor 2. And for preventing this deflection, it is necessary to improve a perpendicularity of the mounting surface (one side surface of the mounting flange 13) of the mounting flange 13 with respect to the rotational center of the hub 8 as well as the surface precision of the mounting surface itself. There respectively exist a plurality of factors giving influences to the perpendicularity and the surface precision. As remarkably influencing factors, they are, as to the perpendicularity, a parallelism between the mounting surface and the raceway surface (the outer ring raceways 11a, 11b and the inner ring raceways 14a, 14b), and as to the surface precision, a heat treatment deformation, and elastic deformation or plastic deformations accompanied with press-fitting of the studs 9 into the mounting holes. Further, for heightening the parallelism of these factors, among the constructive elements of the hub 8, there must be a necessity for setting highly accurately a positional relationship between the mounting surface of the mounting flange 13, the inner ring raceway 14a formed in the outer periphery of the intermediate portion thereof and the stepped part 15 of small diameter formed at the inner end of the same, and configurations and dimensions of these components.
If the precision of shapes and dimensions of the inner ring raceway 14a and the small-diameter stepped portion 15 are enhanced in connection to the mounting surface, the perpendicularity of the mounting surface with respect to the rotational center of the hub 8 may be increased. Further, if removing the heat treatment deformation, elastic or plastic deformation of the mounting surface, the surface precision of the mounting surface may be enhanced.
A technology for preventing the deflection of the flange 13 which conduces to the deflection of the rotor 2 is disclosed, for example, in JP-A-10-217001. However, since the prior art disclosed in this publication precisely finishes a surface which is not originally needed as a reference surface, not only a cost increases, but also no consideration is taken for applying a heat treatment to the respective components. On the other hand, since the inner ring raceway 14a and the small-diameter stepped portion 15 are hardened over the surfaces thereof, a heat treatment such as a high frequency hardening is required. Since the configurations and dimensions of the inner ring raceway 14a and the small-diameter stepped portion 15 are more or less changed due to the heat treatment, according to the prior art disclosed in the publication, it is difficult to sufficiently heighten the precision of each components in the way described above. Moreover, the prior art invention has a structure of fixing to the outer periphery of the hub a pair of inner rings being independent from the hub. Therefore, an error, etc. between the end surface of each inner ring and each inner ring raceway is included as an error of the parallelism between the mounting surface of the mounting flange and the inner ring raceway. Further, since the contact portion between the hub and the inner ring is not processed based on the mounting surface of the mounting flange, it is difficult to sufficiently increase the parallelism between the mounting surface and the inner ring raceway.
Moreover, in order to offset the deflection of the mounting flange 13 and the deflection of the rotor 2 based on the configuration error of the rotor 2 itself, the wheel bearing unit 5 and the rotor 2 are selectively combined, or after the wheel bearing unit 5 and the rotor 2 are combined, the side of the rotor 2 is processed. However, in a case of the former, the selecting operation for combination is troublesome, and in a case of the latter, the mechanical apparatus for processing is complicated and becomes large in size, resulting in an increase in cost in both cases.
Further, in any cases of the conventional structures, any consideration was not taken to the deformation caused in the mounting surface of the mounting flange 13, which is accompanied by the fixation of the stud 9 to the mounting flange 13. A technique giving consideration to the deformation in the mounting surface is known in JP-A-11-294425. According to the conventional technique disclosed in the document, in order to press-fit the base portion of the stud into a mounting hole formed in the mounting flange, it is intended to locate a serration part at the intermediate portion in an axial direction of the mounting hole, which serration is provided at the outer periphery of the base portion of the stud. More precisely, it is intended to secure 13% or more of the thickness of the mounting flange 13 for the distance between both ends of the mounting flange and the end of the serration part.
In such a conventional structure, the deformation of the mounting surface accompanied with press-fitting the stud into the mounting flange is simply considered, and the effect cannot be expected so much. Namely, even if the distance between the mounting flange 13 and the end of the serration part is separated around 13% of the thickness of the mounting flange 13, it is highly probable that the mounting surface is deformed to such a degree as to influence the generation of judder. In contrast, in case the width of the serration part is reduced for increasing the distance, it becomes difficult to surely prevent the stud from being turned.